Controlling Field-of-View of Radio Arrays using Weighting Functions

Transcription

1 Controlling Field-of-View of Radio Arrays using Weighting Functions MIT Haystack FOV Group: Lynn D. Matthews,Colin Lonsdale, Roger Cappallo, Sheperd Doeleman, Divya Oberoi, Vincent Fish

2 Fulfilling scientific promise of future high-sensitivity radio arrays (e.g., SKA) will require the ability to achieve simultaneously: - high angular resolution GHz) - large fields-of-view (~1 o ) - high dynamic range (~10 6 ) One way to meet these goals is with "large-n, small-d" arrays comprising vast numbers of suitably-distributed, smalldiameter antennas, correlated on all baselines: - small dish large intrinsic FOV - excellent u-v plane coverage low sidelobes, high-quality PSF But there are significant challenges...

3 Difficulties: At cm wavelengths, the radio sky is crowded with sources! Sidelobes from out-of-beam sources will limit dynamic range within intended FOV Computational load ~D -6 (Perley&Clark 2003, Cornwell 2004) Removal of unwanted sources and their sidelobes via current techniques (i.e., post correlation) is untenable expected data rates up to ~ PB/s! (Lonsdale 2003) Simulated 1 o 1 o patch of sky at 1.4 GHz; 18'' resolution; F d 10 njy from SKADS Simulated Sky (S 3 ), Oxford University Solution: Correlator FOV Shaping Employ intelligent weighting in frequency/time to limit FOV.

4 "Layers of Attenuation" for an Imaging Array Correlator attenuation Station beam (FIXED) Dirty Beam RMS RMS of attenuated signal Primary beam (FIXED) Lonsdale et al Shaping of correlator FOV can effect an increase attenuation, C(r)

5 Time/bandwidth smearing affects C(r) : 100" from Lonsdale, Doeleman & Oberoi (2005) -100" CLEANed grid of points, no averaging CLEANed, with time & frequency-averaging Note: distorted images & unsubtracted sidelobes Transformation from (f, t) to (u, v) is variable between baselines effective FOV varies between baselines poor image characteristics

6 Correlator FOV Shaping: A Better Approach Concept: Make use of Fourier relationship between measurement (u-v) plane and the sky plane Multiply the sky by a weighting (window) function convolve the u-v plane by Fourier transform of the window function, effectively tailoring the FOV Jinc/top hat function Applying single weighting function in (u, v) plane will impose same FOV on all baselines

7 u-v plane Correlator v t 1 f t 2 f 2 f 1 f 2 f 1 f 2 u t 1 t 2 t f 1 FOV convolution function (same for all baselines) For single baseline: time interval = t 2 t 1 bandwidth = f 2 f 1 Weighted sum yields output visibility Constant size u-v patches map to different-sized f-t sums depending on baseline

8 MIT Array Performance Simulator (MAPS): FOV Simulations General purpose radio array simulator developed at MIT Haystack/SAO (Doeleman, Lonsdale, Cappallo, Bhat, Oberoi, Attridge, Wayth) Correct handling of aperture plane effects (e.g., direction-dependent ionospheric distortions; receptor patterns; phased beam arrays) Incorporates model of correlator data averaging operation to properly treat effects of time and bandwidth smearing; ability to achieve virtually any time or frequency resolution Source attenuation resulting from application of Gaussian FOV weighting from Lonsdale, Doeleman & Oberoi (2005)

9 Limitations/Issues Short baselines need long (f, t) extent; calibration must be stable over Δt and Δt Sets limit on shortest baseline To support FOV weighting, granularity in freq. and time is proportional to baseline length. Sets limit on longest baseline Alternatively data rate depends on baseline length: Rate ~ (b_long/b_short) 2 Due to lowering of data rate on short baselines. To achieve desired reduction in data rate, ultimately will want to apply before data exit correlator - harness high speed computation. Effects of RFI excision require further investigation

10 The Problem of RFI: u-v plane Correlator v t 1 RFI f t 2 f 2 f 1 f 2 f 1 t 1 t 2 u t Each excised time/frequency interval on a given baseline will cause a particular gap in the u-v patch for that baseline convolution function in u-v plane no longer uniform among baselines different FOV shape for each visibility MAPS simulations will be used to characterize RFI effects.

11 (e)merlin: A Test Bed for FOV Shaping Algorithms Range of baseline lengths ideal for FOV algorithm testing Number of baselines small and manageable Data correlation can be performed with Haystack correlator Tests ongoing with data from 4- & 6-element arrays; ν 0 =1650 MHz, Δν=16.0 MHz/512 (V. Fish & D. Foight): 1. Field 1: Two 3C sources separated by ~29 2. Field 2: M31 Results so far: - Both "Jinc" and "Gaussian" weighting functions appear to provide predicted suppression; superior sidelobe rejection compared with time/bandwidth smearing - Technique remains effective even in cases of heavy flagging (up to ~50%) "Jinc" more sensitive to heavy flagging than "Gaussian" (D. Foight 2007)

12 With new WIDAR correlator: Prospects for the EVLA? 100 ms dumps w/ 1 Gb/s ethernet; factor of 10+ improvement possible Δt & Δν control on individual baselines allowed by hardware, but not current software; current Binary Data Format would also need to be updated (M. Rupen) future tests for subset of A-configuration antennas? Possible motivations: way to mitigate effects of wide-field imaging errors? testbed for algorithm development Potential problems: - may not work on the shortest baselines - RFI excision in real time would likely be necessary - $$ + time

13 Issues Currently Being Investigated What is the most effective weighting function to use? Gaussian? Jinc? Other? How will presence of realistic skies affect performance of algorithm? How will use of FOV shaping algorithms affect implementation of various calibration schemes? How will various types of RFI affect algorithm performance? Computational demands? Implementation of FOV shaping in post-correlation hardware? Impact on future array cost equations. Ongoing testing with real (MERLIN) and simulated (MAPS) data at Haystack should provide many new insights